One of the most challenging phases in the life of a dairy cow is the transition from pregnancy to lactation. Lasting a couple of weeks before calving to a couple of weeks after calving, the transition period affects many aspects of the life of the cow.
- Drop in feed intake: A few weeks before calving, cows reduce their feed intake, which is restored a couple of weeks after calving.
- Metabolism: At the same time, the nutrient requirements increase to provide for the fetus and fuel colostrum and milk production. This often leads to energy efficiency and/or mineral imbalances.
- Management: Moving cows to the pre-fresh pen, calving area and then to the fresh group causes stress. In addition, feed changes during this period require adaptation that could worsen the drop in intake.
- Health: Immune responses are weakened because of the shift in metabolism. Inflammation after calving often becomes exaggerated and detrimental. The highest occurrence of diseases (retained placenta, metritis, ketosis, mastitis, displaced abomasum, etc.) is during this period.
The big problem is that cows that fail to overcome those challenges normally have long-term consequences for the entire lactation. We can include producing less milk than their potential and taking longer to get pregnant, making them good candidates for culling.
The transition period is already a difficult time for cows, and when we think it couldn’t get any tougher, summer begins, adding another challenge: heat stress! Heat stress is literally a hot topic, and no one can deny it is a significant problem for Canadian dairy farms. Using the maximum temperature-humidity index (THI) value, which is an important index to determine if cows are in heat stress, it has been established that cows most likely experience 106 days per year of heat stress in Quebec and 115 days per year in Ontario. That is a long period that cows are exposed to heat stress, leading to substantial consequences in performance.
Heat stress adds to an already tough transition period
For transition cows, heat stress elevates the challenges common to this period. Much focus has been given to the effect of heat stress on dry cows and its negative impact on the unborn calf’s performance. In general, dry cows exposed to heat stress end up calving earlier. The shorter gestation length affects how long they stay in the dry pen and receive the close-up diet. Many feeding programs, such as negative dietary cation-anion difference (DCAD) diets, are developed aiming for a three-week close-up program. If calving is anticipated, the duration of the feeding program is reduced, which could influence its efficacy. In addition, a shorter dry period could also limit the period of mammary gland development while dry, which could contribute to reduced performance after calving.
When it comes to fresh cows, they appear more sensitive to the effects of heat stress when compared to mid-lactation or late-lactation cows. The lower feed intake during the transition period can be even worse if the cows are under heat stress. And we can’t blame them – I wouldn’t feel like eating a big Thanksgiving dinner on a 30-degree day under the sun (thankfully, Thanksgiving is in the milder fall weather).
The occurrence of diseases also increases when cows are exposed to heat stress during the transition period. The occurrence of retained placenta or metritis is 1.5 times higher during heat stress, and the occurrence of mastitis is 2 times higher when cows experience heat stress during the transition period.
Another factor that is heavily affected when cows are exposed to heat stress during the transition period is reproduction. We all know that the pregnancy rate goes down in the summer, but we are mistaken if we think this only applies to those cows who are bred in the summer. Those cows bred when summer is gone but spend their transition period during heat stress conditions also have lower chances of reproductive success. Conception is around 7 percentage points lower, while pregnancy loss is 4.3 percentage points higher in cows exposed to heat stress during the transition period. Put together, the odds of getting a cow pregnant soon are very much reduced if they calve during summertime.
Do your cows get heat stressed when you chase fall incentive days?
The discussion around heat stress and its consequences is not new, and many strategies can minimize these effects. However, our farm dynamics change around the summer and fall months because of the incentive days normally given in the fall. It’s a common practice to calve more cows in the summer, and the data from farms across the country shows this trend (Figure 1). We see a rise in the calving numbers around June, July and August. Although the objective is to maximize production to fill the upcoming extra days, many farms end up increasing the number of cows going through their transition period at periods of higher risk of heat stress.
Meanwhile, we also observe a trend of reduced culling when calvings are at peak (Figure 1). Interestingly, numbers from the U.S. point out higher odds of culling within the first trimester of lactation when cows calve during hotter months. Although many herds are trying to fill quota by keeping some cows longer, there is a fine line between selling the most milk possible without decreasing your farm’s profitability. For more insights on managing incentive days, check out the article where Chris Church explored the implications of chasing (or not) these incentive days.
Summer is almost over. What can we do now?
Unfortunately, the best approach to reduce the consequences of heat stress should have been taken before the days were hotter. This includes strategies of heat abatement, such as sprinklers and fans in the barn (and shade if your cows go out) and easy access to clean and fresh water. There are also feed strategies to counter the negative effects. For example, you can increase the density of the ration or use some specific feed additives to overcome lower intake.
However, even as summer wanes, there are still steps you can take. Monitor your herd closely: If you observe that cows are calving earlier than expected, moving cows to the pre-fresh pen twice a week instead of only once can reduce the risk of shorter stays in the close-up group. In addition, pay close attention to intake. Making sure your fresh cows are eating enough, and managing feeding and push-ups to stimulate more frequent meals will help to maintain a steadier intake during the day.
Finally, make sure your strategies to maximize production for the upcoming incentive days are not causing more harm than good. Increasing calving numbers and reducing culling could cause overcrowding in the dry and fresh pens. Overcrowding reduces animal well-being, increases competition and adds more challenges to the already tough transition period.
References omitted but available upon request by sending an email to an editor.
